1. Field of the Invention
The present invention relates to a piezoelectric/electrostrictive structure including a plurality of stacked sheet-shaped piezoelectric/electrostrictive bodies and also relates to a method for manufacturing such a structure by a green sheet-stacking process and a chemical vapor deposition (CVD) process.
2. Description of the Related Art
Printing apparatuses such as printers, facsimiles, and copiers have long used a non-impact process. In particular, most compact printers have recently used such a non-impact process; hence, clear images equivalent to silver photographs can be reproduced on sheets of paper with high-performance inkjet heads. Piezoelectric inkjet heads are typical components of non-impact printing apparatuses. The piezoelectric inkjet heads include actuators including a plurality of piezoelectric/electrostrictive actuating parts and discharge ink using the displacement of the piezoelectric/electrostrictive actuating parts.
FIG. 13 is a perspective view showing an exemplary inkjet head 130. The inkjet head 130 includes a nozzle plate 139 having nozzles 138 and also includes an actuator 131 having ink chambers 135 blocked with the nozzle plate 139. The ink chambers 135, which may be referred to as cells 133, are defined by partitions 136, a substrate 132, and a cover plate 137 and communicatively connected to ink supply channels, which are not shown. The partitions 136 form piezoelectric/electrostrictive actuating parts 134. In the inkjet head 130, the volume of the ink chambers 135 (the cells 133) is varied by applying driving voltages to the piezoelectric/electrostrictive actuating parts 134 (the partitions 136) and ink is therefore discharged from the nozzles 138, whereby printing is performed. Japanese Patent No. 3217006 (hereinafter referred to as Patent Document 1) discloses an inkjet head including an actuator.
There is no limit to the improvement of printing apparatuses for providing clear images. Therefore, for the inkjet head 130, the piezoelectric/electrostrictive actuating parts 134 and the ink chambers 135 need to be more densely arranged. In particular, the ink chambers 135 need to be arranged at 180 dots per inch (dpi) or more. In this case, the pitch between the nozzles 138 is equal to 141 μm (25.4 mm (one inch) per 180 of the nozzles 138). A decrease in the pitch between the nozzles 138 leads to a decrease in the width W of the ink chambers 135 and a decrease in the thickness T of the piezoelectric/electrostrictive actuating parts 134, as is clear from FIG. 13. Therefore, in order not to reduce the volume of the ink chambers 135, the depth (height) D of the ink chambers 135 must be increased by an amount equal to the decrement of the width W thereof. In usual, the depth D thereof is uniform and equal to the height of the piezoelectric/electrostrictive actuating parts 134. The dense arrangement of the piezoelectric/electrostrictive actuating parts 134 and the ink chambers 135 leads to an increase in the aspect ratio D/T of the piezoelectric/electrostrictive actuating parts 134 and an increase in the aspect ratio D/W of the ink chambers 135. The volume of the ink chambers 135 can be maintained by increasing the length of the ink chambers 135 without increasing the width W thereof. However, this technique is not preferable because the area of the actuator 131 is increased.
The actuator 131 having the above configuration can be manufactured by firing a green compact prepared by a green sheet-stacking process. FIGS. 14A to 14C are illustrations showing steps of manufacturing the actuator 131.
Slurry is prepared by mixing a piezoelectric material, a binder, a solvent, a dispersing agent, and an additive such as a plasticizer. Green sheets 16 are then prepared by a doctor blade process or another process using the slurry. The green sheets 16 are punched so as to have a predetermined shape. As shown in FIGS. 14A and the 14B, the resulting green sheets 16 are stacked on the substrate 132 and then pressed, whereby a ceramic green compact 143 is obtained. The ceramic green compact 143 obtained is fired, polarized as needed, wired, and then attached to the cover plate 137, whereby the actuator 131 shown in FIG. 14(c) is obtained. Electrodes may be attached to the actuator 131 as needed.
The actuator 131 prepared by the green sheet-stacking process has a problem in that the interfaces between the green sheets 16 are damaged in some cases when the ceramic green compact 143 is fired. The reason why the damage occurs will now be described with reference to FIGS. 2 and 3.
FIG. 2 is a sectional view showing section A of the ceramic green compact 143 shown in FIG. 14(b). Section A covers some of the piezoelectric/electrostrictive actuating parts 134 and the ink chambers 135. FIG. 3 is an enlarged sectional view showing a part of section A shown in FIG. 2. Each punched green sheet 16 has tapered end faces as disclosed in JP-A-2002-160195 (hereinafter referred to as Patent Document 2). Side faces of the ceramic green compact 143 that are vertical as shown in FIGS. 2 and 3 have notches 25 due to the stacked green sheets 16. With reference to FIG. 3, the stacked green sheets 16 can be displaced as disclosed in Patent Document 2. Therefore, after a pressure P is applied to the stacked green sheets 16 such that the green sheets 16 are unified, the interfaces between the green sheets 16 have unbonded portions 32 created due to a difference in depth between the notches 25. In the actuator 131 prepared by firing the ceramic green compact 143, cracks extend from the unbonded portions 32. Furthermore, since electrode layers are arranged on side faces of the actuator 131 that have the notches 25 and the piezoelectric/electrostrictive actuating parts 134 are distorted by applying voltages between the electrode layers, the actuator 131 is damaged by the cracks and the stress due to the displacement. In particular, actuators prepared by firing green compacts including green sheets and electrode layers placed therebetween have ceramic/metal interfaces and the bonding force between such interfaces is less than that of ceramic/ceramic interfaces. Therefore, such actuators are readily damaged.
An increase in the aspect ratio of the piezoelectric/electrostrictive actuating parts 134 makes such a problem more serious. This is because an excessive increase in pressure applied to the stacked green sheets 16 causes the stacked green sheets 16 to be buckled. Since the buckling strength is inversely proportional to the second power of the height of the piezoelectric/electrostrictive actuating parts 134, an increase in the aspect ratio of the piezoelectric/electrostrictive actuating parts 134 leads to a reduction in buckling strength. Therefore, the pressure applied to the green sheets 16 to be unified must be low. This causes unbonded portions to be created between the green sheets 16 which are not displaced. An increase in the aspect ratio of the piezoelectric/electrostrictive actuating parts 14, that is, a decrease in the width thereof enhances the influence of the unbonded portions on the strength; hence, the actuator 131 is readily damaged. The damage of the actuator 131 leads to the leakage of ink from ink chambers 135 placed in the inkjet head 130 and therefore leads to a serious decline in the reliability of printing apparatuses.